In recent years, digital printing technology has been very vigorously developed. Typical examples of this digital printing technology include those called electrophotographic technology and ink jet technology, and its presence as image forming technology in offices, homes and the like has more and more increased in recent years.
Among these, the ink jet technology has great feature of compactness and low power consumption as a directly recording method. Development of high-quality images is also rapidly advanced by development of micro-nozzles or the like. An example of the ink jet technology includes a method in which an ink fed from an ink tank is evaporated and bubbled by heating it by a heater in a nozzle, thereby ejecting the ink to form an image on a recording medium. Another example includes a method in which an ink is ejected from a nozzle by vibrating a piezoelectric element.
Since aqueous dye solutions are generally used as inks used in these methods, in some cases, bleeding may have occurred when inks of different colors overlap each other, or a phenomenon called feathering may have appeared in a direction of fibers in paper at a recorded portion on a recording medium. In order to improve them, it has been investigated to use pigment-dispersed inks (see U.S. Pat. No. 5,085,698); consequently, ink jet inks that actually use pigment inks are now commonly used.
Many pigments are used as colorants for ink jet recording inks, inks for writing utensils, and the like because of their excellent fastness properties such as water fastness and light fastness. Since the pigment is insoluble in water, it is important to stably disperse it in the form of fine particles in water when it is used for an ink composition. An ink composition using a pigment as a colorant, as described above, is generally prepared by subjecting a mixture composed of the pigment, a liquid medium and a dispersant to a dispersing treatment by means of a dispersing machine or the like to prepare a pigment dispersion liquid and adding various kinds of additives to the pigment dispersion liquid as needed from the viewpoints of easy wetting of the pigment with water and prevention of pigment precipitation. As described above, the pigment is often used in the state of the pigment dispersion liquid upon preparation of ink compositions, and thus various techniques as to pigment dispersion liquid have been developed because they affects the properties of the ink composition to be prepared, in particular.
When an organic pigment or carbon black is used in a pigment ink, however, high definition and high color rendering property cannot be achieved as an ink unless the pigment is very finely and stably dispersed, and the resulting ink may be liable to cause clogging at orifices in some cases unless the dispersion stability of the pigment is excellent. Therefore, for the thermal system among ink jet recording systems, which can provide high-quality images at a high speed but imposes heat on part of the ink, an ink excellent in dispersion stability and re-solubility is preferable.
In addition, ink compositions prepared from pigment dispersion liquids used heretofore cannot realize the glossiness and resistance to bronzing of color images formed with such ink compositions as well as the storage stability of the ink compositions at the same time. More specifically, the ink compositions prepared from the pigment dispersion liquids used heretofore have not been sufficient in glossiness of color images formed, have caused a phenomenon referred to as the so-called bronzing that a recorded surface looks bronze according to viewing angle depending on the particle size distribution of pigment particles ejected, resulting in a failure to realize high image quality, and moreover have not been sufficient in storage stability.
As described in Japanese Patent Application Laid-Open No. 2004-027156, a method, in which the periphery of a pigment is crosslinked with a crosslinkable resin to physically enhance the dispersion stability of the pigment, has been used in recent years.